How will cross-chain trades be done? If I understand correctly, the intention is that inside BitShares system it should be easy to trustlessly trade shares in any DAC, and the protocol is specifically designed to support cross-chain exchanges. For example, if I have BitUSD on BitShares X chain, how I buy shares in DNS chain? Will user interfaces make it straightforward so that users don't have to understand details of the protocol? Will there be special BitShares X chains for trading shares in other DACs?